Updated: Jul 23, 2021
The sea walnut (Mnemiopsis leidyi), or warty comb jelly, is a species of tentaculate ctenophore that, as its name suggests, is shaped like a walnut. It is a stingless jelly-like animal that can grow up to several inches in length. Most sea walnuts are white or transparent, but they are known to be bioluminescent. Rows of cilia, which are similar in appearance to the teeth of a comb, line the sides of the jellys' bodies and create pulsing rainbows as they easily refract light. This often occurs when they are agitated.
The sea walnut is native to the Atlantic Coast of North and South America. It is able to live across a wide spectrum of habitats, with a large range of salinities as well as different temperature conditions.
"Mnemiopsis leidyi is a free-spawning, simultaneous hermaphrodite capable of self-fertilization (Costello, 2001). It possesses gonads containing both the ovary and the spermatophore bunches in their gastrodermis." - Global Invasive Species Database
Reproduction usually occurs during the night when water temperatures range from 66-73 degrees and there are high food concentrations. At optimal conditions, each comb jelly produces 8,000 eggs per spawn. Sea walnuts do not develop distinctive larval and polypoid stages, making them hard to distinguish from each other when they are young. Juvenile sea walnuts can begin reproducing just thirteen days after hatching.
Sea walnuts' diet consists of mainly zooplanktonic, although it varies with their development. Early stages prey on protozoa and microzooplankton, while mature forms feed primarily on crustaceans (often copepods, cladocera), mollusk larvae, eggs, and young fish larvae. Ctenophores like the comb jelly don't sting, however, their lobes possess special adhesive cells called colloblasts that release a sticky, mucus-like substance to trap prey.
What is interesting about the sea walnut is that it is likely the most studied ctenophore genus in the world. This has a lot to do with the fact that they have very strong survival mechanisms. In addition to their aforementioned ability to survive in a range of habitat conditions, they are also known to have cannibalistic tendencies toward their larvae when prey reserves diminish. In addition to their already opportunistic qualities, their cannibalism allows the sea walnut to maintain resilience in the midst of changes and disruptions in the environment.
It is this wide environmental tolerance that makes them a natural enemy, namely, an invasive species:
"Within the Caspian Sea it has been recorded that 2-3,000 eggs are made every day and due to the eggs developing within 20 hours of being laid the species population is able to boom rapidly." - The Liquid Earth
After being accidentally introduced to the Black Sea during the early 1980s, likely with ballast water from the northwestern Atlantic coastal region, it has spread to the Sea of Azov, the Sea of Marmara, the eastern Mediterranean, the Caspian Sea, and the Adriatic Sea. In 2006, it was also first recorded in the North Sea and the Baltic Sea.